Industrial applications such as pulp and paper production, plastic extrusion, conveyor belts and wind power generation are generally characterized by a low-speed and high-torque performance. Permanent magnet (PM) machines with a concentrated winding topology are an attractive alternative for these applications since they are able to provide the desired performance when being direct-driven. A gearbox can be eliminated, which in turn reduces the costs and increases the efficiency.
A concentrated winding topology means that each armature coil is wound around one single stator tooth in an electrical machine. Such winding configuration offers a large reduction of copper material compared with distributed winding topology where the coils are wound in laps enclosing several stator teeth. The coil overhang of the distributed winding topology produces unnecessary copper losses and extends the stator's axial dimension, which reduces torque density (or power density for given speed). The concentrated winding topology thus provides the advantages of reduced total active volume and weight of the machine. The use of less coil material also offers a favourable reduction in copper loss and hence a high torque density motor design can be obtained.
A flux linkage between rotor poles and the coils, i.e. a winding factor, is an important design aspect. The maximum average torque output is directly proportional to the winding factor, a higher winding factor implying a higher output torque for a motor with a given frame size. The distributed winding topology provides a winding factor equal to or nearly equal to the ideal value of one. Concentrated winding topology, on the other hand, typically has a lower winding factor lying within the range of 0.93-0.96. In theory, an ideal winding factor can be easily achieved with a concentrated winding topology by choosing the number of stator teeth being equal to the number of rotor poles, but in practice this causes a severe cogging problem. Therefore, the rotor pole number is typically different from the stator teeth number. In most cases the rotor pole number is less than the stator teeth number, but in the following example, in order to better illustrate the present invention, a conventional machine is chosen to have a rotor pole number which is greater than the stator tooth number.
FIG. 1 shows a conventional PM machine 100 with concentrated armature coils 102, a stator 104 having 18 teeth 106 and the rotor 108 having 20 poles 110 (10 pole pairs). The coils 102 are arranged in two winding periodicities 112, each winding periodicity 112 comprising three stator tooth sections 114 representing three electrical phases, and each stator tooth section 114 comprising three stator teeth 106 in a same electrical phase. The stator teeth 106 are distributed along a circumference of the stator 104 with a uniform tooth span 115 i.e. a uniform angular distance between two adjacent stator teeth 106. The stator teeth 106 are separated from one another by stator slots 118.
A doctoral thesis from Jürgen Friedrich, “Bauformen und Betriebsverhalten modularer Dauermagnetmaschinen”, Universität der Bundeswehr München, Neubiberg 1991, discloses on pages 28-36 PM machines with non-uniform stator tooth patterns. In some embodiments there are intermediate teeth with variable width between the stator teeth, and in other embodiments stator tooth sections in a same electrical phase are separated from one another by widened stator slots. By these measures the winding factor appears to be optimized, but at the same time the non-wound intermediate teeth and the widened stator slots appear to deteriorate the overall torque density. In all embodiments disclosed in this thesis there is only a single winding periodicity.
From the foregoing, the desire to improve the overall torque density of an electrical machine with a concentrated winding topology remains.